Video signals as braodcast by television transmitting stations contain horizontal synchronizing pulses at predetermined and highly accurate time intervals with video information contained between these pulses. These horizontal synchronizing pulses trigger the horizontal sweep circuit of the video receiver. It is possible to record the incoming video information including both horizontal synchronizing pulses and the video picture information on magnetic tape. However, after the incoming video information is recorded on the tape and it is desired to then play back the video tape, the horizontal synchronizing pulses do not always appear at the precise predetermined time at which they occurred when the video signal was broadcast. This variation of the horizontal synchronizing pulses from the time in which they should appear is called time base error, and it may be of the order of several microseconds.
Time base error in a video recording system results from variations in the video tape tension during recording and playback, irregularities in the operation of the tape drive transport elements, and from other causes. Time base error from various sources occurs at certain frequencies and harmonics thereof. It is necessary to correct for the time base error of a tape recorded video program in order to achieve satisfactory receiver performance on playback. This is especially true where the tape was recorded on another video recorder than the one on which playback is accomplished. The result of time base error in a video picture, as seen on the video receiver, is that vertical lines will become wavy or discontinuous.
Expensive professional video recorders utilize a long signal delay line having multiple physical taps at which the signal can be removed. The video signal is removed at the tap which just corrects for time base error. Less expensive recorders which are commercially available use 1 inch wide recording tape which has the video program recorded diagonally across the longitudinal direction of the tape by two magentic transducer heads. These recorders require two drive servo systems, one to drive the tape and a second to drive the rotary mount for the two transducer heads. In these systems, time base error results because of inherent differences between the two transducers, and it is not adequately compensated for in present systems.
A German Patent No. 2,122,592 has described a time base error correction system; however, the present invention has several advantages over the German patent which would not be obvious to those skilled in the art. German Patent No. 2,122,592 uses a feed-forward system to regulate the horizontal synchronizing pulse time to reduce time base error. The present invention, in all embodiments, uses a feedback system which has been found to be more stable. Secondly, in the German patent, a reference signal goes directly to the tape drive motor, where as in the present invention, the video signal is first compared to a reference signal and the resulting phase difference is used to control the tape drive motor's angular speed of rotation. This has again been found to be a more stable technique of correcting for time base error.
The German patent develops the control signal for a bucket brigade by the subtraction of a multiple of the signal frequency from a reference frequency. This method does not correct for constant time base error bias or for very slowly varying time base error. The present invention develops a signal to correct for time base error by comparing the phase of the video signal to a reference signal and using only the phase difference in the correction circuit; therefore, constant time base error bias or very slowly varying time base error can be corrected.